In astrophysics, dark matter reffers to undetectable matter or particles whose presence explains unexpected gravitational effects on galaxies and stars. Various assumptions were made on the composition of dark matter :
Brown dwarf stars,
Black holes, etc
However, the observations (or rather lack of direct observations) would imply a non-baryonic nature (proton, neutro), and thus still unknown nature. According to galaxies formation and evolution models as well as cosmological models, dark matter would represent approximately 30% of the total mass of the Universe.